Conventionally, for purpose of improvement of catalytic conversion efficiency of a catalyst used for purification of exhaust gas, an emission control system for an internal combustion engine includes exhaust-gas sensors (e.g., an air/fuel sensor and an oxygen sensor) that are respectively disposed upstream and downstream of the catalyst in a flow direction of the exhaust gas. The exhaust-gas sensors detect an air-fuel ratio of the exhaust gas or detect whether the exhaust gas is rich or lean, and the emission control system controls the air-fuel ratio by a feedback control based on the outputs of the exhaust-gas sensors.
When the air-fuel ratio of the exhaust gas changes from rich to lean or from lean to rich, an output change of the exhaust-gas sensor (e.g., an oxygen sensor) may lag behind a change of an actual air-fuel ratio of the exhaust gas. Thus, the exhaust-gas sensor may have a room for improvement in detection responsiveness.
For example, as described in Patent Document 1 (JP 8-20414 corresponding to U.S. Pat. No. 4,741,817), at least one of an auxiliary electrochemical cell is incorporated to an inside of a gas sensor such as an oxygen sensor for increase in detection responsiveness.
A deterioration diagnosis technology for a catalyst is known. In the diagnosis technology, for example, a rich combustion control and a lean combustion control are performed alternately. In the rich combustion control, an air-fuel ratio of air/fuel mixture supplied to an engine is controlled to become richer. In the lean combustion control, the air-fuel ratio of the air/fuel mixture supplied to the engine is controlled to become leaner. During a predetermined period of the alternately-performed rich and lean combustion controls, a calculation portion calculates a ratio between an integrated value of output values of a downstream exhaust-gas sensor located downstream of the catalyst and an integrated value of output values of an upstream oxygen sensor located upstream of the catalyst. Subsequently, the integrated value ratio is compared with a predetermined reference value. Accordingly, a deterioration diagnosis of the catalyst is performed, in other words, for example, it is determined whether an oxygen storage capacity of the catalyst is small.
When a detection responsiveness of the downstream exhaust-gas sensor decreases due to time deterioration for example, output behaviors of the downstream exhaust-gas sensor may be similar between when the catalyst is deteriorated and when the catalyst is normal (i.e., not deteriorated) in the deterioration diagnosis. Thus, even when the catalyst is deteriorated, it may be determined that the catalyst is normal (i.e., not deteriorated). Therefore, a reference value for determining the decrease of the detection responsiveness of the downstream exhaust-gas sensor may be necessarily set tightly, and a life time of the downstream exhaust-gas sensor may thereby become short. In other words, the time to exchange the downstream exhaust-gas sensor due to the decrease of its detection responsiveness may become earlier.
In Patent Document 1, the auxiliary electrochemical cell is necessarily incorporated into the inside of the gas sensor. Thus, when the auxiliary electrochemical cell is incorporated into a general gas sensor that does not have an auxiliary electrochemical cell, the general gas sensor may need to be changed greatly in structure. For practical use, the gas sensor may be required to be changed in design, and a manufacturing cost of the gas sensor may be increased.
It is an objective of the present disclosure to provide a deterioration diagnosis device for a catalyst, which is capable of changing an output characteristic of a downstream exhaust-gas sensor without great change in design and cost increase, and capable of limiting a decrease of an accuracy of a deterioration diagnosis due to a decrease of a detection responsiveness of the downstream exhaust-gas sensor.